1. Field of the Invention
The present invention relates to a developing device for developing latent images on a latent image carrier using a two-component developer comprising magnetic carrier and toner used in image forming apparatuses such as copiers, facsimile devices, and printers, and also relates to an image forming apparatus and a process cartridge comprising the same.
2. Description of the Related Art
A developer carrier for carrying developer to the developing region opposite the latent image carrier is provided in this type of developing device. Inside the developing sleeve (hollow body), which is formed, for example, in a cylindrical shape, the developer carrier provides magnetic field generation means for generating a magnetic field for using magnetic force to carry the magnetic carrier in the developer on the peripheral surface of that developing sleeve. Toner is electrostatically adsorbed to the magnetic carrier; the toner is transported to the developing region together with the magnetic carrier carried on the peripheral surface of the developing sleeve; and the toner is supplied to the latent image on the latent image carrier in the developing region. The magnetic field generation means has a plurality of magnetic poles along the rotational direction of the developing sleeve. This type of magnetic field generation means includes devices that use an external magnetic field to magnetize various magnetic pole formation locations of a roller-shaped unit formed in a single body, and devices that support individual magnets on a common support such that those magnetic poles are oriented in a specified direction. Developer carried on the peripheral surface of the developing sleeve by the magnetic force of the magnetic field generation means is transported in the direction of the periphery of the developing sleeve (direction of surface movement) by rotating the developing sleeve.
In this type of developing device, developing consumes the toner in the developer as the developer passes through the developing region. For that reason, peeling the developer off of the developing sleeve after development and returning it to the developer housing unit, and constantly drawing up fresh developer onto the developing sleeve and transporting this to the developing region is important for stable developing performance. Specifically, it is important to prevent recirculation, that is, to prevent developer still on the developing sleeve after having passing through the developing region from being re-transported as is to the developing region.
If developer is recirculated in this way, the concentration of the image formed by the developing device is reduced and there is the risk of creating concentration irregularities, and the like. Consequently, in developing devices of the past, for example, in the conventional developing device disclosed in Japanese Unexamined Patent Application No. 2006-293261, in order to peel the developer after development from off the developing sleeve, magnetic poles of the same polarity were arranged adjacent to each other, and a developer separation area was formed between these magnetic poles, in which peeling force was exerted on the developer on the developing sleeve facing from the surface of the developing sleeve to the direction of separation. According to this conventional developing device, because the developer transported to that developer separation area is removed from the developing sleeve by the peeling force and is incorporated into the developer inside the developer housing unit, recirculation can be prevented.
Moreover, in another conventional developing device, as will be described later while referring to the drawings, in configuring the developer carrier, the widths of the end regions of the magnetic roller were broadened as magnetic generation means. Nonetheless, broadening the widths of the end regions of the magnetic roller in this way posed the problem of enlarging the axial dimensions of the developer roller, thereby making it difficult to produce a compact device as is required for the latest types of image forming apparatuses described above, specifically, for the developing devices therein.
The following describes the reason that the aforementioned recirculation of the developer occurs in the axial direction end regions of the developer carrier (magnetic roller end regions) in the region on the developer carrier corresponding to the magnetic field generation means (called the “region corresponding to the magnetic field generation means” hereinafter).
In the central region of the region corresponding to the magnetic field generation means, axial displacement of the lines of magnetic force generated in the axial direction of the developer carrier (simply called “axial direction” hereinafter) is restricted by the magnetic force of the same polarity in the axial direction end region positions on both sides. As a result, in the axial direction central region in the developer separation region on the developer carrier, the peeling force that is acting on the developer has hardly any components in the axial direction. Consequently, in this central region the peeling force acts effectively on the developer, and the developer can be efficiently removed from the peripheral surface of the developer carrier.
In contrast, for the lines of magnetic force generated in the axial direction end regions of the region corresponding to the magnetic field generation means there is little or no magnetic force of the same polarity to restrict displacement to the outside in the axial direction. For that reason, no equilibrium is obtained between the magnetic force of the same polarity present on the inside in the axial direction and the magnetic force of the same polarity present on the outside in the axial direction, and the lines of magnetic force produced in the axial direction end regions are displaced to the outside in the axial direction. Consequently, in the axial direction end regions inside the developer separation region on the developer carrier, the peeling force (force acted in the direction of separation from the developer carrier) acting on the developer is weak and cannot make the peeling force act effectively on the developer, thereby making it impossible to fully remove developer from the peripheral surface of the developer carrier. This appears to be one of the causes of recirculation of developer on the axial direction end regions inside the region corresponding to the magnetic field generation means.
Moreover, in the developer separation region on the developer carrier, the magnetic component facing to the outside in the axial direction that is generated in those end regions may cause the developer to move to the outside region in the axial direction of the developer rather than to the region corresponding to the magnetic field generation means such that the developer adheres to that outside region. However, developer adhering to the outside region is transported together with the surface movement of the developer carrier, and when passing through the developer separation region on the developer carrier, that developer is again drawn back inside the region corresponding to the magnetic field generation means by the magnetic force of the second magnetic pole. This too appears to be one of the causes generating recirculation of developer in the axial direction end regions within the region corresponding to the magnetic field generation means.
Further, this kind of generation of recirculation of developer in the axial direction end regions within the region corresponding to the magnetic field generation means is only in the part where magnetic poles of the same polarity are adjacent to each other. Specifically, in the part where the magnetic poles of differing polarity are adjacent to each other, the lines of magnetic force that come out from one of the magnetic poles turn in toward the magnetic pole close to it, and even when in the axial direction end regions, there are hardly any lines of magnetic force that face to the outside in the axial direction.
Taking this into consideration, in the present invention magnet members are provided in the part outside the magnetic field generation means, and magnetic fields are generated that displace to the inside in the axial direction of the developer carrier the lines of magnetic force that pass through the axial direction end regions within the developer separation region on the developer carrier and that cause peeling force. The orientation of the lines of magnetic force in the axial direction end regions within the developer separation region on the developer carrier can thereby become very nearly orthogonal to the axial direction. Consequently, the peeling force in these end regions is improved, and therefore the peeling force can be effectively applied to the developer even in these end regions, and developer can be efficiently removed from the peripheral surface of the developer carrier. As a result, the axial direction end regions that produce recirculation of developer in the region corresponding to the magnetic field generation means can be reduced in width (length in the axial direction of the developer carrier). Even if the width of the region corresponding to the magnetic field generation means is shorter, the same effective developing width can thereby be realized without generating image concentration irregularities caused by recirculation of developer.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Unexamined Patent Application No. 2006-065217, Japanese Unexamined Patent Application No. 2006-184524, and Japanese Unexamined Patent Application No. 2001-042649.